A major research objective of this grant was the identification of the specific peptide strip, of (if multilocular) of the specific amino acid residues, of the gamma 1 and gamma 2 heavy chains that mediate cell-cell attachments by reacting with complementary receptors of mononuclear cells. An unexpected means for achieving this stems from very recent observations (initiated by Dr. Alper and reported in Section IV) employing systems in which C3b is bound to cross-linked dextran beads; after removing all unbound protein and then hydrolyzing the beads with dextranase, the only proteins recoverable were four separable fragments of C3b, one of which was the active subunit. The isolation of this subunit not only has simplified the problem of identifying the binding residues, but has provided a derivative method for isolating the cell receptors for C3b. Comparable procedures for isolating the active binding residues of gamma 1 and gamma 3 heavy chains, and thereafter for isolating their receptors from mononuclear cells are in progress. Procedures for isolating viable, functional monocytes, lymphocytes and granulocytes in suspension (without inflicting the gross artifacts induced by interfacial density-gradient techniques, as are routinely employed by others) have proved successful, and have permitted observations of what appears to be the natural morphologic appearances, volumes, and surface configurations of these human cell-types. The striking changes induced by altering temperature and other physiologic factors, under conditions precluding the deformities caused by surface-contact or by non-homologous protein or cells, indicate that many of the various cell responses, attributed by others to differing, specific functional properties of subcategories of lymphocytes and monocytes, are the results of non-specific reactions. The nature of human cytologic responses to specific stimuli is now possible, and is in progress using homologous red cells to which specific homologous immunoglobulins have been attached.